It is known to detect cracks in metallic materials for instance in steel production. Inductive techniques have for instance been used for this purpose. When using an inductive technique a current is induced in the metallic material, e.g. a slab or a metal sheet, by means of a time-varying magnetic field generated by a transmitter coil fed with a likewise time-varying current. When the induced current encounters a crack in the metallic material, the crack constitutes an obstacle to the induced current. As a result, the crack alters the induced current at the crack as compared to a metallic material without a crack. The altered current provides a change in the magnetic field around the current. The change in the magnetic field is measured by a receiver coil, whereby it can be determined that a crack is present in the inspected surface portion of the metallic material.
When using inductive techniques for crack detection in metallic materials, the surface of the metallic material is typically searched by means of coils being moved across that portion of the surface which is to be inspected. GB 2 401 947 discloses an apparatus and for inspection of critical surfaces on aircraft engine disks. One embodiment disclosed in GB 2 401 947 is depicted in FIG. 5. This embodiment comprises a transmitter coil 38 that is in the form of a right-angled triangle. Receiver coils 32-1 and 32-2 are arranged along respective adjacent sides 38-1 and 38-2 of the right angle of the triangular transmitter coil 38. The sides 38-1 and 38-2 of the triangle induce orthogonal currents in metallic material that is being inspected for cracks. Thereby cracks having different orientation may be detected by the receiver coils 32-1 and 32-2. Crack inspection is conducted by moving the apparatus along the surface of the aircraft engine disk.
EP 1 033 571 A2 discloses a probe which includes a first substrate in or on which a probe coil is arranged. A second probe coil at least partially overlaps the area covered by the first probe coil, as viewed from above. The second coil is arranged in or on a second substrate. A stack of further substrates may be provided for further probe coils. At least one of the substrates is a film substrate made of flexible material. The first and second coils are used for detecting cracks or faults with mutually differing orientation in the component being tested. A photolithographic method of manufacture is also claimed.
U.S. Pat. No. 4,584,746 discloses a device for detecting cracks in steel slabs leaving continuous casting. The devices comprises two crack detecting units placed on either side of the passage of a slab leaving continuous casting and two descaling and cooling units located upstream of the detection units. Each detection unit comprises a mobile frame and a bead which slides vertically with respect to the frame and which bears one vertical sensor and three horizontal sensors. The sensors are induction coils placed in a container which is surrounded by a forwardly open metal casing. One application of the device is the automatic switching of defective slabs towards a reprocessing unit.
In production processes of metallic products it has proved to be difficult to utilise equipment of the above-mentioned type. Firstly such crack inspections may take too long time for fitting into a production flow. Secondly the mechanical devices required for inspection become too expensive and too sensitive for the conditions, e.g. high temperatures in a continuous casting process, which often prevail during production.